Can't have <library> and/or conditional requirements in a composed propagated feature.

A possible [solution](https://stackoverflow.com/a/45428334/14396720) does not work. I got `error: expansions of composite features result in conflicting values`.

I want to conditionally use a library (with alternatives) for targets. I don't want to add conditions everywhere but prefer to create a propagated feature. However, I got `error: Recursion in main target references`. The question is also mentioned [here](https://stackoverflow.com/q/64613164/14396720).

Start by reproducing the composed propagated feature using the conditional requirements described in the issue and compare the conflicting-values and recursion errors. Read the linked Stack Overflow solution and question for the intended usage and investigate the B2 feature and target-requirement entry points. Done means a conditional library choice with alternatives can be expressed through a propagated feature without requiring conditions everywhere or triggering recursion.

Written by the indexing model from the issue text.